This invention relates broadly to the art of implantable medical devices and, more particularly, to apparatus having first and second electrodes which deliver an electrical potential to body tissue in a common mode configuration and which operate as bipolar sensors when switched to a sensing mode. The invention is particularly useful for cardiac pacing systems, defibrillation apparatus, heart stroke volume rate adaptive pacemakers, cardioversion devices and the stimulation and measurement of other physiological events.
With respect to this invention's contribution to the art, it is helpful to first present some background on the state of the art. Present heart pacing systems pace and sense through the same electrodes, making it difficult to verify capture of cardiac tissue. The after-potentials following a pacing impulse are much greater than the evoked potentials of the myocardium so immediate detection of electrograms from the pacing electrode is very difficult, if not impossible, due to total swamping of the evoked response. Prior art systems have attempted to reduce the effects of such after-potentials by various methods.
For example, U.S. Pat. No. 4,343,312 to Cals, et al., discloses a physiological stimulating system, which in a pacemaker embodiment, delivers signals constituted of a series of alternating polarity pulses of respective time durations and signal levels in an attempt to minimize the resulting polarization at the point of delivery of such signals. Cals is trying to overcome the large after-potential signal at the electrode which occurs immediately after delivery of a negative-going stimulus pulse. Since it takes some time for this after-potential signal to dissipate, the signal has the effect of masking the evoked response which occurs shortly after application of the negative-going stimulus pulse employed by Cals.
Similarly, U.S. Pat. No. 3,924,641 to Weiss provides a biphasic current stimulation system in a attempt to avoid unfavorable interactions between electrodes and tissues caused by pulses of one polarity. A similar scheme is also taught by UK Patent Application GB 2,193,101A to Napholz entitled "Pacing Pulse Compensation". Napholz teaches a pacemaker which generates a triphasic stimulus wherein the first and third phases are positive pulses and the second is the negative stimulus. European patent application publication number 0308536 filed Sept. 24, 1987 entitled "Pacemaker Having Programmable Configuration" discloses a pacemaker having a programmable configuration including a plurality of solid state switches controlled by programmed data stored in memory. During the pacing mode of operation a first reference electrode is switchably connected to the most positive battery potential and during a sensing mode of operation, the first reference electrode is switchably connected to -0.5 volts. Such prior art techniques for eliminating after-potentials in a cardiac stimulation system are very complex. All of them require a significant delay before sensing of the evoked response can be accomplished in order to allow neutralization of the signal. If the delay is too long the evoked response may be lost entirely.
Other prior art techniques use similar electrode configurations and, instead of applying biphasic pulses, first apply a stimulation pulse and then tie the sensing electrodes together after the body tissue is stimulated. These schemes also require a significant delay to allow neutralization of the signal before sensing of the evoked response can be accomplished.
The present invention overcomes the perceived disadvantages of prior art devices in providing a system for a cardiac pacemaker including common mode stimulation apparatus with bipolar sensing apparatus so as to allow sensing of the evoked cardiac response almost immediately after application of the stimulation signal from a pacing device.
The pacing scheme provided by the present invention is a general method that permits capture verification through the use of bipolar sensing and pacing in parallel through both sensing electrodes to an indifferent electrode. The pacing and sensing method may be implemented using a switch or a pair of diodes as is shown in FIGS. 1 and 2. The electrode configuration for this pacing method may vary, ranging from a bipole located on a catheter to an epicardial bipole electrode or electrode pair. As described above, the success of this pacing method requires that the two bipolar electrodes be designed to have similar pacing after-potentials so that the common pacing signal can be rejected by the bipolar sensing amplifier. This apparatus is improved further by electrode design techniques which reduce electrode after-potentials such as is discussed in the assignee's application for a "Porous Electrode With Enhanced Reactive Surface", filed Mar. 20, 1989, Ser. No. 07/325/764 which is incorporated herein by reference.
As will be appreciated by those skilled in the art, the scope of the invention extends beyond the field of pacemakers. For example, the teachings of the invention are also applicable to defibrillation apparatus, heart stroke volume sensing apparatus, cardioversion devices and measurement and stimulation of other physiological events. Generally, the invention can be employed in virtually any situation wherein a pulsed electrical potential is applied to a human body and one desires to measure the body's response to the application of the pulsed electrical potential.